附录A 快速参考指南¶

// 变量声明
val immutable = "不可变"      // 只读
var mutable = "可变"          // 可修改

// 空安全
var nullable: String? = null  // 可为null
var nonNull: String = "值"    // 不可为null

// 安全调用
val length = nullable?.length  // null时返回null
val len = nullable?.length ?: 0 // null时使用默认值

// 非空断言（谨慎使用）
val risky = nullable!!.length  // null时抛出NPE

// 基本函数
fun greet(name: String): String {
    return "Hello, $name"
}

// 单表达式函数
fun greet(name: String) = "Hello, $name"

// 默认参数
fun greet(name: String, greeting: String = "Hello") = "$greeting, $name"

// 命名参数
greet(name = "Alice", greeting = "Hi")

// 高阶函数
fun operate(a: Int, b: Int, op: (Int, Int) -> Int) = op(a, b)
val sum = operate(1, 2) { x, y -> x + y }

// 数据类
data class User(val id: String, val name: String)

// 使用
val user = User("1", "Alice")
val copy = user.copy(name = "Bob")
val (id, name) = user  // 解构

// 密封类
sealed class Result<out T>
data class Success<T>(val data: T) : Result<T>()  // 泛型<T>：类型参数化
data class Error(val exception: Exception) : Result<Nothing>()

// 单例
object DatabaseHelper

// 伴生对象
class MyClass {
    companion object {
        const val VERSION = "1.0"
        fun create() = MyClass()
    }
}

// 启动协程
val scope = rememberCoroutineScope()
scope.launch { /* 后台任务 */ }
val deferred = scope.async { /* 返回结果 */ }
val result = deferred.await()

// Flow
val flow = flow {
    emit(1)
    emit(2)
}

// 收集
flow.collect { value -> println(value) }

// 操作符
flow.map { it * 2 }.filter { it > 2 }.collect { }

// 文本
Text("Hello", style = MaterialTheme.typography.bodyLarge)
Text(text = "Hello", fontSize = 16.sp, color = Color.Red)

// 按钮
Button(onClick = { }) { Text("Click") }
OutlinedButton(onClick = { }) { }
TextButton(onClick = { }) { }
IconButton(onClick = { }) { Icon(Icons.Default.Add, null) }

// 图片
Image(painter = painterResource(R.drawable.image), contentDescription = null)
AsyncImage(model = url, contentDescription = null)

// 输入框
var text by remember { mutableStateOf("") }
OutlinedTextField(
    value = text,
    onValueChange = { text = it },
    label = { Text("Label") }
)

// Column - 垂直排列
Column(
    modifier = Modifier.fillMaxSize().padding(16.dp),
    verticalArrangement = Arrangement.spacedBy(8.dp),
    horizontalAlignment = Alignment.CenterHorizontally
) {
    Text("Item 1")
    Text("Item 2")
}

// Row - 水平排列
Row(
    modifier = Modifier.fillMaxWidth(),
    horizontalArrangement = Arrangement.SpaceBetween,
    verticalAlignment = Alignment.CenterVertically
) {
    Text("Left")
    Text("Right")
}

// Box - 层叠
Box(modifier = Modifier.fillMaxSize()) {
    Text("Bottom", modifier = Modifier.align(Alignment.BottomCenter))
}

// Lazy列表
LazyColumn {
    items(dataList) { item ->
        ItemComposable(item)
    }
}

// remember
val count = remember { mutableStateOf(0) }
Text("${count.value}")

// rememberSaveable（配置变更保留）
var text by rememberSaveable { mutableStateOf("") }

// derivedStateOf
val isEnabled by remember { derivedStateOf { count.value > 0 } }

// ViewModel
class MyViewModel : ViewModel() {
    private val _uiState = MutableStateFlow(UiState())
    val uiState: StateFlow<UiState> = _uiState.asStateFlow()
}

@Composable
fun MyScreen(viewModel: MyViewModel = hiltViewModel()) {
    val state by viewModel.uiState.collectAsState()
}

Modifier
    .fillMaxWidth()              // 填充宽度
    .fillMaxSize()               // 填充整个空间
    .width(100.dp)               // 固定宽度
    .height(50.dp)               // 固定高度
    .size(100.dp)                // 固定宽高
    .padding(16.dp)              // 内边距
    .padding(horizontal = 8.dp)  // 水平内边距
    .background(Color.Red)       // 背景色
    .border(1.dp, Color.Gray)    // 边框
    .clip(RoundedCornerShape(8.dp)) // 圆角
    .clickable { }               // 点击

// 依赖配置
dependencies {
    implementation("androidx.core:core-ktx:1.13.1")
    testImplementation("junit:junit:4.13.2")
    androidTestImplementation("androidx.test.ext:junit:1.1.5")
}

// 版本管理
android {
    compileSdk = 35

    defaultConfig {
        minSdk = 24
        targetSdk = 35
        versionCode = 1
        versionName = "1.0.0"
    }
}